The present invention relates to circular saws, having fine-pitch uniformly shaped non-metallic teeth, for cutting thin materials.
Background: Cutting Tool Hardness
Since the beginning of the Bronze Age, toolmakers have sought to improve the durability and functionality of tools by modifying their cutting edges. Early processes included work-hardening of bronze, and adding steel edges to iron implements. That process continues to this day as new, super-hard materials are developed and new applications are found for older ones. In general, the ideal cutting tool surface should combine abrasion-resistance (hardness) with shock-resistance (toughness). (Of course there are many other relevant properties, including yield strength, rigidity, temperature limits, corrosion resistance in some applications, etc.)
Solutions evolved as described in the Neibl U.S. Pat. No. 907,167(1909) and the Blum U.S. Pat. No. 1,535,096(1926), where a band of very hard steel was welded to another supporting band of softer steel and teeth were cut into the hard steel band. Whitaker, U.S. Pat. No. 1,130,649, (1916) and the Napier U.S. Pat. No. 1,352,140(1921), approach the same problem by applying special heat treatment and tempering to the cutting edge.
U.S. Pat. No. 1,919,358 to Bem suggested applying Stellite to the edge of a saw body, and then grinding teeth into that material. In the Bern patent, a hard facing material (in the form of a welding rod) was applied to the edge of a saw blade body by melting with an acetylene torch. Although Stellite is now an obsolete material, "hardfacing" materials are often applied by torch or spraying to the wear-edges of plows, bulldozer blades and rock drill stems. It is tempting to assume that these techniques could be used to build up a useful hard layer on a lathe tool or a saw blade, as proposed in the Bem patent. Unfortunately, such coatings suffer from high porosity, very low strength, and poor impact properties, rendering them unfit as cutting tools.
Following the development of tungsten carbide materials produced by powdered metal technology by Schroeter, U.S. Pat. No. 1,594,615 (1925), tungsten carbide moved steadily into tooling applications.
Background: Carbide-Toothed Circular Saws
Cutting tools (especially woodworking tools) often use inserted teeth of a material which is harder than the hardest of steels. The most common material used for this is a "cemented carbide," which typically includes small grains of tungsten carbide bonded into a matrix with a metal (typically cobalt). (Because the strength and hardness of the matrix are derived from the grains of tungsten carbide, such cemented carbides are often referred to simply as "carbide.") Such "carbide" saw tips have a hardness of about 92(Rockwell A). FIG. 3 shows a typical configuration of a section of a carbide tipped circular saw. Typically the main part of the saw blade 310 is a steel plate, and the carbide teeth 320 are brazed onto the leading edge of tooth profiles 330 which are cut out from the steel plate.
Some firms manufacture only the steel bodies of circular saws, which are hardened, tempered and finished in every way except for tipping, and are then sold to other saw manufactures who specialize in carbide tipping. Other firms manufacture the complete saws including both the steel bodies and the installed tips. In either case, the same standard carbide tips are used in the fabrication of the blades. The steel bodies are normally made of high-carbon alloy tool steel, then a pocket is ground into the periphery of the saw body to accommodate the carbide tips. The tips may be 1/4 to 3/8 inches long, 0.062 to 0.093 inches thick and from 0.10 to 0.375 inches wide, depending on the width of the finished saw blade.
The normal industry practice is to affix the carbide tips to the steel bodies by means of brazing, typically with silver bearing brazing material. While satisfactory for most applications, the brazed joints limit the operating temperature and loads (particularly lateral loads) under which the blade can operate.
Additionally, the thickness of the carbide and the size of the brazed area provided by the seat pocket have a large bearing on the integrity of the joint. However, the same factors that favor a strong brazed joint also limit the number of carbide inserts that can be placed along the cutting edge of the saw. This, in turn, ultimately limits the minimum spacing between adjacent teeth, the number of teeth on the saw, and the number of teeth per inch of periphery. For example, carbide-tipped ten-inch blades are currently available with up to 100 teeth, but no more. This implies that the pitch of a conventional carbide-tipped saw cannot be less than one centimeter.
In the woodworking industry, carbide tipped saws are typically 8 to 20 inches in diameter. Depending on their function, the 8 inch blades may have between 24 and 48 teeth, and the larger saws 60 to 100 teeth. For cutting non-ferrous metals, the number of teeth is typically between 24 and 80 for saws ranging from 8 to 18 inches in diameter. However, saws with greater tooth density (i.e. more teeth per inch) would be required to produce superior finishes and to cut thin materials.
The geometry of brazed carbide circular saw construction limits tooth density to a maximum of about 10 teeth per inch of saw diameter, i.e., 100 teeth for a 10 inch saw. Some applications require a higher tooth density than carbide tipped saws can provide. In woodworking, these applications include the cutting of plywood and veneers where fine finish is required and splintering is unacceptable. Tooth density of at least twice that available from carbide tipped saws is required to cut thin metals, including thin wall tubing and extruded shapes. In addition, fine tooth saws leave less burr on cut surfaces.
More recently, the same technology has been applied to use polycrystalline diamond inserts which are dimensionally similar to and applied in the same manner (by brazing) as carbide. However, the same limitations on tooth pitch will still apply.
When faced with a problem requiring more saw teeth than carbide tipping can provide, the manufacturer has two choices. He can choose a steel saw and forego the benefit of the longer lifetime of carbide (which is about 4:1 over high speed steel), or he can select a solid carbide saw. The steel saws are relatively inexpensive but dull rapidly. Solid carbide saws have superior wear properties, but are brittle and are not available in sizes greater than 6 inches in diameter. Moreover, solid carbide saws are very expensive, typically 15 times that of a steel saw, as shown in FIG. 4, where the comparative costs of a steel blade versus a solid carbide blade are plotted at various diameters.
Background: Grit-Surfaced (Non-Toothed) "Saws"
A common type of cutting tool is a circular blade which does not have shaped teeth at its edge, but which is simply coated with a diamond grit. Such cutting tools are commonly referred to as diamond "saws," but in fact they do not perform the same type of material-removal action as is performed by a saw with shaped teeth. A saw with shaped teeth, when it is operating correctly, will carve off chips of material. By contrast, a grit-coated blade will have more of a scraping or abrasive action. (See generally Jim Effner, Chisels on a wheel (1992 ); and Peter Koch, Utilization of Hardwoods Growing on Southern Pine Sites (1985 ); both of which are hereby incorporated by reference.) A cutting action is greatly preferable for many applications, to produce a cleaner cut, lower temperature, and lower power requirements.
Background: Cutting Thin Materials
Sawing action is smoothest when there is always at least one tooth in the cut. For cutting very thin materials, this requires a very fine tooth pitch. If the tooth pitch cannot be made as small as the material thickness, it should still be made as small as possible.
For example, for cutting aluminum extrusions with ten-inch blades, blades with up to 300 teeth are often used; however, these are steel blades, and become dull relatively quickly. Similarly, artificial veneers of hard composites, and thin carbon-fiber composites, are extremely abrasive and are best cut by fine-tooth saws.
Background: Circular Saws with Tooth Segments
Large circular saws of the type used in sawmills (e.g. 20 to 60 inches in diameter) have long been designed with removable teeth. In a sawmill environment, this is very advantageous, because a damaged tooth can be replaced or resharpened quickly and easily without dismounting the entire blade. A variation of this, as described for example in U.S. Pat. No. 3,633,637 is to use removable multi-tooth segments for sawmill blades.
Background: Saw-Blade Tensioning
A circular saw appears to be simply a flat plate of metal with teeth, but this appearance is very deceiving. In use the saw blade will be rotating at a high speed (e.g. 3600 RPM for a 16" blade), and may be heated at its edge by the cutting operation (particularly if the saw has begun to get dull). These forces would tend to expand the circumference of a thin flat plate. Thus if the sawblade were simply a thin flat plate it might expand around its circumference, and might wobble. (Wobbling is very undesirable, since it makes the cutting action more imprecise, wastes material, slows cutting, and increases the wear on the saw blade, arbor, and bearings.) To avoid this, a substantial amount of tension is built into the blade's structure. Correct tensioning is particularly important with saw blades which are very thin. See U.S. Pat. No. 5,522,283, and the references cited therein, all of which are hereby incorporated by reference. The correct use of saw blade tensioning is critical, especially for large saw blades; and the elasticity of the steel used in the saw plate is part of the equation. That is, a material with a different elasticity would have a different relation between stress and expansion, and hence the balance between built-in stress and thermally-induced stress would be different. Another relevant characteristic of steel is that plastic deformation can be used to induce the desired built-in stress without fracturing or significantly weakening the material. Other materials (even very strong materials) would have different balances of these characteristics, and might not be as useful for saw blades.
Saw Blade With Segmented Multi-Tooth Super-Hard Close-Pitch Inserts
The present inventors have realized that there is no substitute for a superhard material in applications for cutting small aluminum extrusions, artificial veneers of hard composites, and similar materials. The present application discloses a saw blade (preferably a circular saw blade) with segmented multi-tooth close-pitch inserts of an extremely hard material, such as a cemented carbide. This permits super-hard materials to be used for extremely fine-pitch saw blades. To achieve this, segments of a superhard material are brazed to the rim of an alloy plate (preferably a steel plate) to form the complete circular saw blade. Preferably an intermediate layer, such as copper, is brazed between the superhard material and the plate.
An added advantage in assembling the rim from segments is that it is possible in an alternative method of assembly to first grind teeth into the segments and then coat the teeth with a vapor deposited diamond surface. The segments would then be brazed to the body of the saw, and the diamond surfaces on the teeth would be lapped to their final configuration.